1. Field of the Invention
Embodiments of the present invention relate, in general, to range localization and more particularly to identifying the location of an object within three dimensional space by correlating a single transmission from multiple receivers.
2. Relevant Background
Accurately determining relative position is critical to successful implementation of a variety of technology. What technology is employed and how it is used often hinges on to what degree of accuracy the position of the relative components can be determined. The range of such a need is immense and varies from the need to position a read/write head over the correct track as it flies some 2/1,000,000 of an inch above a magnetic disk as the disk revolves some 10,000 revolutions per minute, to enabling an aircraft to fly halfway around the world and land within inches of its intended destination without the aid of any visual references.
Accurate positional awareness has traditionally relied on a mechanical coupling between two or more components. Indeed that is how the computer storage read/write head can be positioned much closer to a magnetic disk (much closer than the thickness of a human hair) without fear of making contact to that disk. But such mechanical linkage presents numerous limitations. The most obvious of which is the inability to navigate from one location to another or to simply judge how far a certain object is.
In World War II a major breakthrough occurred with respect to position awareness. That invention was the use of radio waves to determine the distance to an object and the rate at which that object was moving. More commonly known as Rate and Distance and Range (“Radar”) revolutionized many aspects of everyday life. Certainly the ability to position an aircraft or ship for military purposes was a distinct advantage but as the technology evolved, the same concept found its way into many aspects of everyday life. For example weather forecasting and speed traps find their roots in the Radar technology of World War II.
As technology continued to evolve, a new means of determining relative position was developed by understanding slight variations in how long it takes a radio signal to reach a receiver. Radio waves travel at substantially the speed of light. Therefore, strong enough signals from vastly different locations, will reach the same location at different times. When the time differences from three different transmitters from known locations are compared, a triangulation of sorts can be calculated and the position of its intersection can be determined. Unfortunately, the speed of light requires that the relative distances needed to appreciably measure temporal differences in the signal be very large. The Global Positioning Satellite (“GPS”) system solves that challenge by placing several very large signal transmitters high in earth orbit.
The Global Positioning System (GPS) is a locational and navigational system that allows users to pinpoint a place with great accuracy. The current GPS system makes use of signals transmitted by some of the 24 dedicated satellites circling the globe in precisely defined orbits. Using the satellites as reference points, GPS receivers calculate their positions based on the difference in arrival time of signals from the different satellites. Although GPS was initially developed for the U.S. military to guide missiles to targets, it is now routinely used for air traffic control systems, ships, trucks and cars, mechanized farming, search and rescue, tracking environmental changes, and more.
As mentioned above, GPS is a space-based satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites. The GPS program provides critical capabilities to military, civil and commercial users around the world and is the backbone for modernizing the global air traffic system.
The GPS project was developed in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. To determine a location on the earth, a GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include the time the message was transmitted and the satellite position at time of message transmission.
The receiver uses the messages it receives to determine the transit time of each message and computes the distance or range to each satellite. These distances along with the satellites' locations are used to compute the position of the receiver. A satellite's position and range define a sphere, centered on the satellite, with radius equal to the range. The position of the receiver is somewhere on the surface of this sphere. Thus with four satellites, the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver would be at a precise intersection of the four surfaces.
One of the most significant error sources is the GPS receiver's clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the range, are very sensitive to errors in the GPS receiver clock; for example an error of one microsecond (0.000001 second) corresponds to an error of 300 meters (980 ft). This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work; however, manufacturers prefer to build inexpensive GPS receivers for mass markets. This dilemma is resolved by taking advantage of the fact that there are four ranges.
It is likely that the surfaces of the three spheres intersect, because the circle of intersection of the first two spheres is normally quite large, and thus the third sphere surface is likely to intersect this large circle. If the clock is wrong, it is very unlikely that the surface of the sphere corresponding to the fourth satellite will initially intersect either of the two points of intersection of the first three, because any clock error could cause it to miss intersecting a point. On the other hand if a solution has been found such that all four sphere surfaces at least approximately intersect with a small deviation from a perfect intersection then it is quite likely that an accurate estimation of receiver position will have been found and that the clock is quite accurate.
The current GPS system is comprised of three segments; the space segment, the control segment and the user segment. The space segment (SS) is as one might imagine composed of the orbiting GPS satellites. The orbits are centered on the Earth, not rotating with the Earth, but instead fixed with respect to the distant stars. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees apart which, of course, sum to 360 degrees.
The control segment is composed of a master control station (MCS), an alternate master control station, four dedicated ground antennas and six dedicated monitor stations. The flight paths of the satellites are tracked by dedicated monitoring stations. Then the agency responsible for the satellites contacts each GPS satellite regularly with navigational updates using dedicated or shared ground antennas. These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model.
The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user.
The vast distance between the GPS satellite transmitters and the receivers enables relatively simple processing to determine range differences and thus a location. The system depends on the knowledge of each satellites precise location and a precise time indication of when the signal was transmitted. A need exists to determine a precise location of an object without the limitations of the prior art. These and other deficiencies of the prior art are addressed by one or more embodiments of the present invention.